If you run into traffic and get hit by a car, the conversation is about that, not "under thinking" because the impact is painful and obvious.
However, it's more subtle for someone to notice that it takes you 30 minutes to cross the street because you're way too cautious, and the impact of that ("you could be doing a lot more with your life than standing here") is less obvious.
- Look before you leap. - Measure twice, cut once. - Stay silent and have people think you're a fool, open your mouth and confirm it.
I'm sure there are many more common idioms pointing towards the idea of "think before you act".
- Don't dive head first into water.
- Measure nine times, cut once.
And a random saying I just remembered: "Autumn has the eyes of a summer and the teeth of a wolf."
Edit:formatting
- being late (acting in "autumn") ...looks/feels similar to
- being on time (seizing the moment at the right point, in "summer")
...but has a nasty bite?
We do this all the time. We just don't use the word "underthink". "You didn't think this through." "It sounds like you haven't gotten to the details yet." "Are you sure that's how it works?" "You need to start thinking things through." "You have a brain. Use it." "Pull your head out your ass." I think all of those phrases are suggesting the same thing underthink would.